Many strains of medically important bacteria have become increasingly resistant to currently available antibiotics. Healthcare associated infections caused by multi-drug resistant pathogens are particularly vexing. Worldwide, millions suffer from antibiotic-resistant infections, which results in a huge cost to the healthcare system. The need for new antibiotics has become a critical, unmet need in the medical community (Infectious Diseases Society of America, 2010).
Lantibiotics, an important class of antibiotics with potential clinical relevance (reviewed in Smith & Hillman, (2008) Curr. Opin. Microbiol. 11:401), acquired their name because of the characteristic lanthionine rings that are present. Lantibiotics are also known to have various unusual amino acids such as 2,3-didehydroalanine (Dha), 2,3-didehydrobutyrine (Dhb), S-amino vinyl-D-cysteine (AviCys), aminobutyrate (Abu), 2-oxopropionyl, 2-oxobutyryl, and hydroxypropionyl. Hasper et al. (2006) Science 313, 1636-1637. Mutacin 1140 (“MU1140”) rings A and B (see FIG. 1A), the lipid II binding domain, is similar to nisin, a well-known lantibiotic produced by Lactococcus lactis that has been used in the food industry for over 50 years. It was discovered that both nisin and MU1140 abduct lipid II from the site of new cell wall synthesis, ultimately causing cell death. Smith et al. (2008) Biochemistry 47:3308-3314.
Particular features of lantibiotics, such as their novel and diverse mechanisms of action and, in instances where it has been studied (Chatterjee et al., (2005) Chem Rev. 105:633), the difficulty of sensitive bacteria to acquire resistance, have aroused considerable interest in these molecules as potential therapeutic agents. Until now, organic synthesis of lantibiotics also has been thwarted because of the complex intertwined ring structures found in these highly unusual peptide molecules (e.g., Rings C/D of MU1140 in FIG. 1A).
The problem of synthesizing intertwined macrocyclic rings characteristic of lantibiotics has recently been solved. See, U.S. Pat. No. 7,521,529; U.S. Publ. No. 2009/0215985. Differentially Protected Orthogonal Lanthionine Technology (DPOLT) is a peptide synthesis platform technology that has excellent potential for the cost-effective, large scale manufacture of all known lantibiotics. The crux of DPOLT involves manufacture of two novel, differentially protected lanthionine (Alanine-S-Alanine) building blocks for intertwined ring construction. The use of these building blocks, in combination with standard solid and/or solution phase peptide synthesis chemistry, is essential for synthesis of the intertwined rings.
MU1140 can be synthesized by a particular strain of the oral microorganism Streptococcus mutans. Smith et al. (2000) Eur. J. Biochem. 267:6810-6816. When laboriously produced through large scale fermentation methods and purified using stepwise precipitation, chromatographic, and crystallization methods, it demonstrated a submicromolar minimum inhibitory concentration (MIC) for all Gram positive bacteria against which it was tested. Ghobrial et al. (2009) International Journal of Antimicrobial Agents 33:70-74. The study also demonstrated that MU1140 is bactericidal against S. pneumonia and multi-drug resistant strains of S. aureus, bacteriostatic against vancomycin-resistant Enterococcus faecium (VREF), and had no activity against Gram-negative bacteria or yeast. See id. The study showed that MU1140's time-kill profiles for selected pathogens were similar to those of vancomycin, one of the currently used antibiotics of last resort. See id. It has a novel mechanism of action which involves binding to and abducting lipid II essential for cell wall biosynthesis. Hasper et al., (2006) Science, 313:1636; Smith et al., (2008) Biochem. 47:3308. It had low cytotoxicity in vitro, low toxicity when administered via an intravenous route in murine models, and it was distributed into all body compartments. Ghobrial et al., J. Pharm. Sci. Epub: Dec. 28, 2009, DOI 10.1002/jps.22015. Demonstration of efficacy was achieved in a pilot study in which 60 times the LD50 of Staphylococcus aureus was administered in a rat peritonitis model. Development of significant resistance was not observed during repeated subculture of S. aureus or Streptococcus pneumoniae in medium containing sub-lethal concentrations of MU1140. Ghobrial et al. (2009) International Journal of Antimicrobial Agents 33:70-74. The basis for this observation may be due, in part, to the fact that the molecular target, lipid II, is evolutionarily ancient and highly conserved throughout the bacterial kingdom, indicating that mutations which alter its structure and/or function may be prohibited. The molecular structure of MU1140 contains four macrocyclic rings (see FIG. 1A), each of which contains a lanthionine or methyllanthionine residue. This odd chemical feature is likely to be important in the resistance of MU1140 to hydrolytic degradation, as has been reported. Hillman et al., Infect. Immun. 44:141 (1984). Resistance to hydrolysis may also be, in part, a reflection of the unusual, horseshoe-shaped three dimensional structure of MU1140. Smith et al. (2003) Biochem. 42:10372-10384. Based on these and other studies, MU1140 has the potential to replace current, failing drugs of last resort and serve in the treatment of problematic infections caused by Gram positive bacteria such as methicillin resistant S. aureus (MRSA), vancomycin resistant Enterococci (VRE), and Clostridium difficile (C. diff).